COMPARATIVE EVALUATION OF

CORROSION RESISTANCE OF EPOXY

TREATED REINFORCEMENT AND

ENAMEL TREATED REINFORCEMENT

By

Prof.(Dr) Pravat Kumar ParhiProfessor, Civil Engg Deptt, College of Engineering & Technology, Bhubaneswar

Mr. Soumya Ranjan MohantyM Tech Scholar, Civil Engg. Deptt, College of Engg. & Technology, Bhubaneswar

Comparative Evaluation of

Epoxy Treated Reinforcement

and Enamel Treated

Reinforcement

What is corrosion of steel ?

Corrosion is the chemical or electrochemical

reaction between a material, usually a metal, and

its environment that produces a deterioration of

the material and its properties.

For steel embedded in concrete, corrosion results

in the formation of rust which has two to four

times the volume of the original steel.

Corrosion also produces pits or holes in the

surface of reinforcing steel, reducing strength

capacity as a result of the reduced cross-sectional

area.

Can corrosion be avoided ?

Yes if:

Concrete is always dry, then there is no H2O to form rust. Also

aggressive agents cannot easily diffuse into dry concrete.

Concrete is always wet, then there is no oxygen to form rust.

Cathodic protection is used to convert all the reinforcement into

a cathode using a battery. This is not easy to implement

because anodic mesh is expensive, and this technology is not

easy to install and maintain.

A polymeric coating is applied to the concrete to keep out

aggressive agents. These are expensive and not easy to apply

and maintain.

A polymeric coating is applied to the reinforcing bars to protect

them from moisture and aggressive agents. This is expensive

but effective.

How we can prevent corrosion?

Watertight concrete and proper cover

Non-chloride accelerators

Cathodic protection

Sealers

Polymer concrete overlays

Silica-fume concrete overlays

Epoxy-coated rebar

Stainless steel rebar

Galvanized steel reinforcement

Glass-fiber-reinforced-plastic rebar

Corrosion-inhibiting admixture

Alternative to epoxy coating ENAMEL COATING

Three different types of enamel

coatings

1.Reactive enamel

2.Pure enamel

3. Double enamel

The reactive enamel was obtained by

combining pure enamel with calcium silicate

(cement) at a 1-to-1 ratio by weight.

The double enamel was composed of an inner

layer of pure enamel and an outer layer of

reactive enamel.

OBJECTIVES

The main objective of this study is to characterize the relative

corrosion resistance of three enamel coatings that have been

applied to deformed steel reinforcing bars through a non-

electrostatic dipping process.

(1) evaluate the relative corrosion performance of the newly developed

reactive enamel coating when embedded within a highly alkaline

environment through designing, constructing, and monitoring of several

reinforced concrete ponding specimens;

(2) evaluate the relative corrosion performance of the three enamel

coatings when placed within a humid, sodium chloride (NaCl)

contaminated environment with an elevated air temperature;

(3) quantify each coating’s overall ability to postpone the onset of

corrosion when placed within a corrosion cell;

(4) conduct a forensic investigation upon the reinforced concrete

ponding specimens;

RESEARCH PLAN

PART-1:

Ponding Test specimens were constructed to evaluate the

corrosion resistance of the enamel coating within a cementitious

environment. As a baseline for comparison, both uncoated, enamel

coated and epoxy-coated steel reinforcement were also tested.

The test consisted of subjecting a total of 25 ponding specimens to

a continuous two week wet / one week dry cycle, for a period of

54 weeks. Concrete resistivity and half-cell potential readings were

carried out every 6 weeks over the course of the testing period.

Upon completion of the test, each reinforced specimen was then

forensically evaluated.

Using both the AASHTO T259 and ASTM C1543 standard as

guidelines, ponding specimens were constructed to evaluate the

corrosion resistance of the 50/50 enamel coating within a

cementitious environment. As a baseline for comparison, both

uncoated and epoxy-coated steel rebar were also tested.

The typical reinforced ponding

specimen and formwork.

Ponding specimen during either

the wet or dry phase of testing

Ponding test procedure

The specimens were subjected to a series of consecutivewet/dry cycles.

The wet phase of a wet/dry cycle lasted for a total of twoweeks and consisted of placing 2 litres of saltwater within aspecimen’s reservoir. The saltwater remained within aspecimen’s reservoir during the entire two weeks and consistedof distilled water with 5 percent sodium chloride (NaCl) byweight.

The dry phase of a wet/dry cycle began when the saltwatercontained within the specimen’s reservoir was removed and thespecimen was then permitted to air dry for a period of oneweek.

The wet/dry cycling of the specimens began directly aftercollecting the baseline resistivity and corrosion potentialmeasurements for each specimen. Baseline readings wereconducted within the first week after a group of specimens hadreached an age of 28 days.

Concrete resistivity and corrosion potential readings were then

Two types of Measurements

Concrete Resistivity Measurements. The resistivity of each specimen was measured every two weeks with

the use of a Multi-meter, an analyzing instrument which had a fixed electrode spacing of 2 in. (5.1 cm) and a nominal alternating current AC output of 180 μA at a frequency of 50 Hz. The equipment had an impedance of 10 MΩ and an operating range of 0 to 99 kΩcm with a 1 kΩcm resolution. The equipment was portable and required two AA batteries. Resistivity measurements began immediately after a wet phase of testing had been completed.

Corrosion Potential Measurements. The corrosion potential of the rebar embedded within a specimen was

measured immediately after the specimen’s resistivity readings were recorded. Using the Multi-metre equipment, which had an operating range of ±999 mV, the corrosion potential at three locations along the length of each embedded bar was measured. These locations were spaced 6 in. (15 cm) on centre and were offset a distance 3 in. (7.6 cm) from a specimen’s side.

The overall average resistance of each

specimen type throughout the testing period.

An average representation of the final

corrosion potential of each specimen group at

week 54

Findings of Ponding Test

Concrete Resistivity Measurements. The significance of these values is a relative

indication of the corrosion resistance of the concrete/rebar system for each coating

type. With the reinforced specimens having been constructed with the same concrete

and steel reinforcement, the discrepancy within the resistivity readings is most likely

attributed to the coating applied to the embedded reinforcement. This result would

indicate that the epoxy coating provided the greatest resistance to the applied

electrical current, while the uncoated bar provided the least resistance. The 50/50

enamel-coated bars provided a degree of resistance between that of the epoxy and

uncoated bars.

Corrosion Potential Measurements. Taking into account these results, it was found

that the corrosion protection provided by the epoxy coating was jeopardized when

damaged, while the corrosion protection provided by the 50/50 enamel was unaltered

when damaged. Although the corrosion protection of the 50/50 enamel coating was

unaffected by the areas of damage, the coating consistently provided a lower level of

protection when compared to that of the intentionally damaged epoxy-coated bars.

The final set of corrosion potential measurements indicated a “high > 90% ”

probability that the reinforcement contained within each specimen group was actively

corroding. With a severe chance that the reinforcement contained within the two

50/50 enamel groups and the uncoated group had begun to corrode.

PART-2:

A salt spray test was used to rapidly assess the relative

corrosion performance of the three enamel coatings

along with a standard epoxy coating. The test consisted

of subjecting a total of 64 specimens to a series of

wet/dry cycles for a period of 12 weeks. After testing, the

uniformity of each coating, as well as the steel-coating

bond along both the deformed and smooth bars, was

evaluated through visual and microscopic cross-

sectional examination.

A modified ASTM B117 salt spray test was used to

assess the corrosion resistance of three enamel coating

configurations along with a standard epoxy coating.

Salt Spray test

A standard salt spray test method specifically designed to evaluate the relative

corrosion resistance of various metals and/or coatings. Today, salt spray chambers

are designed according to the ASTMB117 standard and are automated to maintain a

specified environment within the chamber.

A salty fog is injected into the enclosed chamber through a nozzle or atomizer

centrally located along the chamber’s floor.

The distribution of the salt fog throughout the chamber shall have a fallout rate such

that 2.0 to 4.0 ml of solution. 1.0 to 2.0 ml per second is collected upon a horizontal

surface measuring 80 cm2.

Specimens within the chamber shall be oriented at an angle of 15° to 30° from the

vertical and positioned in such a manner that prevents the specimens from

contacting one another.

A specimen’s exposure to the salt fog shall be unobstructed. Solution that

accumulates inside the chamber may be disposed of through a drain positioned

within the chamber’s floor. Prior to opening the chamber, a ventilating system may be

used to expel any salt fog lingering within the chamber; however, opening of the

chamber shall be held to a minimum.

The validity of the test may also be established by examining standard test

specimens, of known performance, alongside specimens whose performance has not

Representation of a salt spray

chamber

The specimen layout within the

salt spray chamber

Testing & procedure

During the twelve weeks of testing, the set of 64 specimens was broken up into two groups of 32 specimens. Group 1 contained all of the deformed bars, and Group 2 contained all of the smooth bars.

The two groups of specimens were transferred from one condition to the other on Monday, Wednesday, and Friday of each week. The total duration of the salt spray test was 2000 hours with each of the two groups spending half of the time in a dry environment and the remaining 1000 hours in a salty fog (wet) environment.

After a group had spent 72 hours within the wet environment, the group would spend the following 72 hour phase in the dry environment. This cycling was maintained throughout the 2000 hours of testing and resulted in each group spending an equal amount of time in both the wet and dry environments.

The condition of a typical deformed 50/50 enamel-

coated specimen after the fifth and twelfth week of

testing.

(a) Fifth week. (b) Twelfth week.

The areas along a deformed double enamel-coated

specimen showing various amounts of corrosion.

a “Minor.” b

“Moderate.

The areas along a deformed pure enamel-coated specimen

showing various amounts of corrosion.

a “Minor.” b “Significant.”

The deformed epoxy-coated salt

spray specimens after testing.

Conclusions

1. The 50/50 enamel coating is more susceptible to impactdamage than that of the epoxy coating.

2. When embedded in concrete, the 50/50 enamel coating canreduce the electrical conductivity of a steel bar. However, theinsulating properties of the coating are lower than that of anepoxy coated steel bar.

3. An area of damage, measuring approximately 0.2 in.2 (1.3cm2) in size, will have no influence upon a 50/50 enamel-coated bar’s performance during a ponding test.

4. Of the three enamel coatings, the 50/50 enamel coatingprovides the least amount of protection to the underlyingsteel, while the double enamel provides the highestamount of protection, and the pure enamel provides adegree of protection between the double and 50/50enamel coatings.

5. The overall performance of the three enamel coatings depended

significantly on the minimum thickness of each coating.

6. The excellent bond created between the steel reinforcement and both pure

and double enamel coatings actively prevents corroding areas from travelling

along the steel-coating interface (i.e., no undercutting); whereas, the epoxy

coating is unable to do so.

7. When undamaged and properly applied, both pure and double enamel

coatings can protect steel reinforcement from chloride induced corrosion;

whereas, the 50/50 enamel coating cannot.

Reference

ASTM A 775 (2007). Standard Specification for Epoxy-Coated Steel ReinforcingBars. American Society of Testing and Materials, West Conshohocken, PA.

ASTM B 117 (2009). Standard Practice for Operating Salt Spray (Fog) Apparatus.American Society of Testing and Materials, West Conshohocken, PA.

ASTM C 876 (2009). Standard Test Method for Corrosion Potentials of UncoatedReinforcing Steel in Concrete. American Society of Testing and Matierals, WestConshohocken, PA.

ASTM A 934 (2007). Standard Specification for Epoxy-Coated Prefabricated SteelBars. American Society of Testing and Materials, West Conshohocken, PA.

ASTM C 1543 (2009). Standard Test Method for Determining the Penetration ofChloride Ion into Concrete by Ponding. American Society of Testing and Materials,West Conshohocken, PA.

Doppke, T. and Bryant, A. (1983). "The Salt Spray Test: Past, Present, andFuture."Proc. of the 2nd Automotive Corrosion Prevention Conference, 57-72.

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